229 research outputs found
An extreme, blueshifted iron line profile in the Narrow Line Seyfert 1 PG 1402+261; an edge-on accretion disk or highly ionized absorption?
We report on a short XMM-Newton observation of the radio-quiet Narrow Line
Seyfert 1 PG 1402+261. The EPIC X-ray spectrum of PG 1402+261 shows a strong
excess of counts between 6-9 keV in the rest frame. This feature can be modeled
by an unusually strong (equivalent width 2 keV) and very broad (FWHM velocity
of 110000 km/s) iron K-shell emission line. The line centroid energy at 7.3 keV
appears blue-shifted with respect to the iron Kalpha emission band between
6.4-6.97 keV, while the blue-wing of the line extends to 9 keV in the quasar
rest frame. The line profile can be fitted by reflection from the inner
accretion disk, but an inclination angle of >60 deg is required to model the
extreme blue-wing of the line. Furthermore the extreme strength of the line
requires a geometry whereby the hard X-ray emission from PG 1402+261 above 2
keV is dominated by the pure-reflection component from the disk, while little
or none of the direct hard power-law is observed. Alternatively the spectrum
above 2 keV may instead be explained by an ionized absorber, if the column
density is sufficiently high (N_H > 3 x 10^23 cm^-2) and if the matter is
ionized enough to produce a deep (tau~1) iron K-shell absorption edge at 9 keV.
This absorber could originate in a large column density, high velocity outflow,
perhaps similar to those which appear to be observed in several other high
accretion rate AGN. Further observations, especially at higher spectral
resolution, are required to distinguish between the accretion disk reflection
or outflow scenarios.Comment: Accepted for publication in ApJ (18 pages, 5 figures, 1 table
The X-ray Spectral Properties and Variability of Luminous High-Redshift Active Galactic Nuclei
We perform a detailed investigation of moderate-to-high quality X-ray spectra
of ten of the most luminous active galactic nuclei (AGNs) known at z>4 (up to
z~6.28). This study includes five new XMM observations and five archived X-ray
observations (four by XMM and one by Chandra). We find that the X-ray power-law
photon indices of our sample, composed of eight radio-quiet sources and two
that are moderately radio loud, are not significantly different from those of
lower redshift AGNs. The upper limits obtained on intrinsic neutral hydrogen
column densities, N_H<~10^{22}-10^{23} cm^{-2}, indicate that these AGNs are
not significantly absorbed. A joint fit performed on our eight radio-quiet
sources, with a total of ~7000 photons, constrains the mean photon index of z>4
radio-quiet AGNs to Gamma=1.97^{+0.06}_{-0.04}, with no detectable intrinsic
dispersion from source to source. We also obtain a strong constraint on the
mean intrinsic column density, N_H<~3x10^{21} cm^{-2}, showing that optically
selected radio-quiet AGNs at z>4 are, on average, not more absorbed than their
lower-redshift counterparts. All this suggests that the X-ray production
mechanism and the central environment in radio-quiet AGNs have not
significantly evolved over cosmic time. The mean equivalent width of a putative
neutral narrow Fe Ka line is constrained to be <~190 eV, and similarly we place
constraints on the mean Compton reflection component (R<~1.2). None of the AGNs
varied on short (~1 hr) timescales, but on longer timescales (months-to-years)
strong variability is observed in four of the sources. In particular, the X-ray
flux of the z=5.41 radio-quiet AGN SDSS 0231-0728 dropped by a factor of ~4
over a rest-frame period of 73 d. This is the most extreme X-ray variation
observed in a luminous z>4 radio-quiet AGN.Comment: 10 pages (emulateapj), 5 figures. Accepted by Ap
The XMM-Newton view of the relativistic spectral features in AXJ0447-0627
The XMM-Newton observation of the optically Type 1 AGN AXJ0447-0627 (z=0.214)
unambiguously reveals a complex, bright and prominent set of lines in the 4-8
keV rest frame energy range. Although, from a phenomenological point of view,
the observed properties can be described by a simple power law model plus 5
narrow Gaussian lines (at rest frame energies of nearly 4.49, 5.55, 6.39, 7.02
and 7.85 keV), we find that a model comprising a power law (Gamma of the order
of 2.2), a reflected relativistic continuum, a narrow Fe I Kalpha line from
neutral material as well as a broad Fe Kalpha relativistic line from a ionized
accretion disk represents a good physical description of the data. The ''double
horned'' profile of the relativistic line implies an inclination of the
accretion disk of the order of 45 degree, and an origin in a narrow region of
the disk, from R_in of the order of 19 GM/c^2 to R_out of the order of 30
GM/c^2. The narrow Fe I Kalpha line from neutral material is probably produced
far from the central black hole, most likely in the putative molecular torus.
Although some of these properties have been already found in other Type 1 AGN
and discussed in the literature, at odd with the objects reported so far we
measure high equivalent widths (EWs) of the observed lines: nearly 1.4 keV for
the ``double horned'' relativistic line and nearly 0.4 keV for the narrow line.Comment: 16 pages, 3 figures, Latex manuscript; accepted for publication in
Ap
Simultaneous EUVE/ASCA/RXTE Observations of NGC 5548
We present simultaneous observations by EUVE, ASCA, and RXTE of the type~1
Seyfert galaxy NGC 5548. These data indicate that variations in the EUV
emission (at keV) appear to lead similar modulations in higher
energy (\ga 1 keV) X-rays by 10--30 ks. This is contrary to popular
models which attribute the correlated variability of the EUV, UV and optical
emission in type~1 Seyferts to reprocessing of higher energy radiation. This
behavior instead suggests that the variability of the optical through EUV
emission is an important driver for the variability of the harder X-rays which
are likely produced by thermal Comptonization. We also investigate the spectral
characteristics of the fluorescent iron K line and Compton reflection
emission. In contrast to prior measurements of these spectral features, we find
that the iron K line has a relatively small equivalent width
(eV) and that the reflection component is consistent
with a covering factor which is significantly less than unity (--0.5). Notably, although the 2--10 keV X-ray flux varies by % and the derived reflection fraction appears to be constant throughout our
observations, the flux in the Fe~K line is also constant. This behavior
is difficult to reconcile in the context of standard Compton reflection models.Comment: 13 pages, 6 figures, LaTeX, uses emulateapj.sty and apjfonts.sty,
submitted to Ap
Variable Iron K-alpha Lines in Seyfert 1 Galaxies
We find that variability of the iron K-alpha line is common in Seyfert 1
galaxies. Using data from the ASCA archive for objects that have been observed
more than once during the mission, we study the time-averaged spectra from
individual observations, thereby probing variability on timescales that range
from days to years. Since the statistics of the data do not warrant searches
for line variability in terms of a complex physical model, we use a a simple
Gaussian to model the gross shape of the line, and then use the centroid
energy, intensity and equivalent width as robust indicators of changes in the
line profile. We find that ~70% of Seyfert 1s (ten out of fifteen) show
variability in at least one of these parameters: the centroid energy,
intensity, and equivalent width vary in six, four, and eight sources
respectively. Due to the low S/N, limited sampling and time averaging, we
consider these results to represent lower limits to the rate of incidence of
variability. In most cases changes in the line do not appear to track changes
in the continuum. In particular, we find no evidence for variability of the
line intensity in NGC 4151, suggesting an origin in a region larger than the
putative accretion disk, where most of the iron line has been thought to
originate. Mkn 279 is investigated on short timescales. The time-averaged
effective line energy is 6.5 keV in the galaxy rest frame. As the continuum
flux increases by 20% in a few hours, the Fe K line responds with the effective
line energy increasing by 0.22 keV (~10,500 km s^-1). Problems with the ASCA
and Rosat calibration that affect simultaneous spectral fits are discussed in
an appendix.Comment: 26 pages, 30 figures, accepted for publication in the Astrophysical
Journa
Fe XXV and Fe XXVI Diagnostics of the Black Hole and Accretion Disk in Active Galaxies: Chandra Time-Resolved Spectroscopy of NGC 7314
We report the detection of Fe XXV and Fe XXVI emission lines from a
Chandra HETGS observation of the Seyfert~1 galaxy NGC 7314, made simultaneously
with RXTE. The lines are redshifted (cz ~ 1500 km/s) relative to the systemic
velocity and unresolved. We argue that the lines originate in a near face-on
(<7 degrees) disk having a radial line emissivity flatter than r^-2. Line
emission from ionization states of Fe in the range ~Fe I up to Fe XXVI is
observed. The ionization balance of Fe responds to continuum variations on
timescales less than 12.5 ks, supporting an origin of the lines close to the
X-ray source. We present additional, detailed diagnostics from this rich data
set. These results identify NGC 7314 as a key source to study in the future if
we are to pursue reverberation mapping of space-time near black-hole event
horizons. This is because it is first necessary to understand the ionization
structure of accretion disks and the relation between the X-ray continuum and
Fe K line emission. However, we also describe how our results are suggestive of
a means of measuring black-hole spin without a knowledge of the relation
between the continuum and line emission. Finally, these data emphasize that one
{\it can} study strong gravity with narrow (as opposed to very broad) disk
lines. In fact narrow lines offer higher precision, given sufficient energy
resolution.Comment: Accepted for publication in the Astrophysical Journal. 30 pages, six
figures, five of them color. Abstract is abridge
Broad-band X-Ray Spectra of the Black Hole Candidate GRO J1655-40
We present broad-band (2 keV to 2 MeV) X-ray spectra of GRO J1655-40, a
luminous X-ray transient and occasional source of relativistic radio jets,
obtained with RXTE and OSSE. In one observation, the luminosity is found to be
18% of the Eddington limit, which is one of the highest luminosities ever
observed from GRO J1655-40. For this observation, we find that an adequate fit
is obtained when a broad iron line and a reflection component are added to a
model consisting of a power-law plus a soft excess component. The 95%
confidence lower limit on the rms line width is 0.86 keV. The power-law
component has a photon index of 2.72 and extends to at least 800 keV without a
cutoff. After this observation, a significant drop in the (5-12 keV)/(1.5-5
keV) hardness ratio occurred on a timescale less than 2 hours. From an RXTE
observation of GRO J1655-40 made after the hardness transition, we find that
the power-law index is harder (2.415 +/- 0.011), the flux of the power-law
component is lower, and the total luminosity is 10% of the Eddington limit. The
change in the power-law component is consistent with the correlation between
the spectral index and power-law flux previously reported for GRO J1655-40.Comment: 20 pages, 5 figures, Accepted for publication in the Astrophysical
Journa
An Observational Determination of the Bolometric Quasar Luminosity Function
We combine a large set of quasar luminosity function (QLF) measurements from
the rest-frame optical, soft and hard X-ray, and near- and mid-infrared bands
to determine the bolometric QLF in the redshift interval z=0-6. Accounting for
the observed distributions of quasar column densities and variation of spectral
energy distribution (SED) shapes, and their dependence on luminosity, makes it
possible to integrate the observations in a reliable manner and provides a
baseline in redshift and luminosity larger than that of any individual survey.
We infer the QLF break luminosity and faint-end slope out to z~4.5 and confirm
at high significance (>10sigma) previous claims of a flattening in both the
faint- and bright-end slopes with redshift. With the best-fit estimates of the
column density distribution and quasar SED, which both depend on luminosity, a
single bolometric QLF self-consistently reproduces the observed QLFs in all
bands and at all redshifts for which we compile measurements. Ignoring this
luminosity dependence does not yield a self-consistent bolometric QLF and there
is no evidence for any additional dependence on redshift. We calculate the
expected relic black hole mass function and mass density, cosmic X-ray
background, and ionization rate as a function of redshift and find they are
consistent with existing measurements. The peak in the total quasar luminosity
density is well-constrained at z=2.15+/-0.05. We provide a number of fitting
functions to the bolometric QLF and its manifestations in various bands, and a
script to return the QLF at arbitrary frequency and redshift from these fits,
as the most simple inferences from the QLF measured in a single band can be
misleading.Comment: 24 pages, 11 figures. Submitted to ApJ. A routine to return the QLF
from the fits herein is available at
http://www.cfa.harvard.edu/~phopkins/Site/qlf.htm
The nature of a broad line radio galaxy: Simultaneous RXTE and Chandra HETG observations of 3C 382
We present the results from simultaneous chandra and rxte observations of the
X-ray bright Broad-Line Radio Galaxy (BLRG) 3C 382. The long (120 ks) exposure
with chandra HETG allows a detailed study of the soft X-ray continuum and of
the narrow component of the Fe Kalpha line. The rxte PCA data are used to put
an upper limit on the broad line component and constrain the hard X-ray
continuum. A strong soft excess below 1 keV is observed in the time-averaged
HETG spectrum, which can be parameterized with a steep power law or a thermal
model. The flux variability at low energies indicates that the origin of the
soft excess cannot be entirely ascribed to the circumnuclear diffuse emission,
detected by chandra on scales of 20-30 arcsec (22-33 kpc). A narrow (sigma<90
eV) Fe Kalpha line (with EW< 100 eV) is observed by the chandra HEG. Similar
values for the line parameters are measured by the rxte PCA, suggesting that
the contribution from a broad line component is negligible. The fact that the
exposure is split into two observations taken three days apart allows us to
investigate the spectral and temporal evolution of the source on different
timescales. Significant flux variability associated with spectral changes is
observed on timescales of hours and days. The spectral variability is similar
to that observed in radio-quiet AGN ruling out a jet-dominated origin of the
X-rays.Comment: 19 pages, 10 figures, 3 tables, accepted for publication in Ap
Suzaku Observations of Active Galactic Nuclei Detected in the Swift/BAT Survey: Discovery of "New Type" of Buried Supermassive Black Holes
We present the Suzaku broad band observations of two AGNs detected by the
Swift/BAT hard X-ray (>15 keV) survey that did not have previous X-ray data,
Swift J0601.9-8636 and Swift J0138.6-4001. The Suzaku spectra reveals in both
objects a heavily absorbed power law component with a column density of NH =~
10^{23.5-24} cm^{-2} that dominates above 10 keV, and an intense reflection
component with a solid angle >~ from a cold, optically thick medium. We
find that these AGNs have an extremely small fraction of scattered light from
the nucleus, <~ 0.5% with respect to the intrinsic power law component. This
indicates that they are buried in a very geometrically-thick torus with a small
opening angle and/or have unusually small amount of gas responsible for
scattering. In the former case, the geometry of Swift J0601.9-8636 should be
nearly face-on as inferred from the small absorption for the reflection
component. The discovery of two such objects in this small sample implies that
there must be a significant number of yet unrecognized, very Compton thick AGNs
viewed at larger inclination angles in the local universe, which are difficult
to detect even in the currently most sensitive optical or hard X-ray surveys.Comment: 4 pages, 3 figures, accepted for publication in ApJ Lette
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